CI: set runner label-to-image mapping in config.yml

Move ubuntu-latest → runner-base:py3.12-node22 mapping into
act-runner-config.yml so it is respected at runtime. The
GITEA_RUNNER_LABELS env var is only honoured on first registration
and was being ignored after the runner's .runner file was created.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infra/act-runner-config.yml

@@ -8,6 +8,10 @@ log:
 runner:
   capacity: 2   # max concurrent jobs
   timeout: 30m
+  # Maps runs-on labels to Docker images for job containers.
+  # Change this (and delete /data/.runner) to switch the base image.
+  labels:
+    - "ubuntu-latest:docker://git.fabledsword.com/bvandeusen/runner-base:py3.12-node22"
 
 cache:
   enabled: true